The invention relates to a method for soldering components onto printed circuit boards using a soldering mechanism having at least one stirrup electrode heatable by electrical resistance heating.
Soldering mechanisms having stirrup electrodes secured to a soldering stirrup holder and heatable by electrical resistance heating are disclosed, for example, by DE-A-31 49 236, by DE-A-28 18 958 and corresponding U.S. Pat. No. 4,255,644, or by EP-A-0 011 046. Such soldering mechanisms having two stirrup electrodes lying in parallel opposite one another or having four stirrup electrodes lying opposite one another in pairs are particularly utilized for soldering multi-pole electronic components such as micropacks, flat packs and the like.
DE-A-31 49 236 discloses a tool consisting of at least two vertical and parallel bars made of a conducting material and electrically insulated from each other. The bottom ends of the bars are screwed for holding at least two thermodes. Each thermode is made of a material with low electrical resistance and low thermal inertia, so it is rapidly heated by the passage of an electric current through the thermode. The preferred tool uses four vertical bars located at the corners of a square and carrying four thermodes. Each thermode consists of a U-shaped strip with a narrow web and widened legs with holes for screws so the thermodes can be attached to the bars. The tool has four small Au-plated brass columns forming a rectangular or square section at the lower ends of which are thermodes, connected between adjacent columns and cut from a single plate, in the form of a U with widened ends for good electrical contact with the columns. The cut faces of the plates contact the elements to be welded. Columns diagonally opposed are connected to one pole of the power source. The tool has a simple design but thermodes are resistant to flexure. Also, minimum contact surface with welded parts, accurate control of temperature distribution, and partly replaceable construction are further advantages.
EP-A-0 011 146 discloses an apparatus where in the working surface of a bit, electric current flows along at least two separate paths which are joined together at both ends. The current in the two paths is preferably switched in parallel; and the paths may form a ring surface on the working end of the bit. The current may be fed into two zones separated from each other by 180 degrees on a ring bit, and then flows out of the ring via two more zones which are also separated from each other by 180 degrees and are 90 degrees from the current inlet zones. The apparatus is useful for the soldering of flat contact tags on an electronic component to a carrier or board.
When such soldering mechanisms are additionally equipped with a suction pipette arranged centrally between the stirrup electrodes, the mechanism can be utilized for the automatic equipping of the printed circuit boards with the components to be soldered thereon. When equipping according to what is referred to as the "pick and place" principle, the combined equipping and soldering head moves over the delivery modules of the components and over the printed circuit board arranged in the equipping region, whereby the suction pipette picks up the respective component from a delivery module and places the component on the printed circuit board in the prescribed equipping position.
After the respective component has been placed down, the soldering stirrup holder is then lowered, so that an optimally reliable contact derives between the working surfaces of the stirrup electrodes, the terminal legs of the component, and the interconnects, or, respectively, terminal pads of the printed circuit board both before the soldering process as well as over the entire duration of the soldering process. The suction pipette can remain on the component until the soldering process is ended; it can press the component against the soldering location on the printed circuit board with a limited force.
In the soldering process triggered by the electrical resistance heating of the stirrup electrodes, the solder provided for the soldering, arranged on or integral with the terminal pads, melts. This melting leads to what is referred to as a sagging or re-sagging of the stirrup electrodes. The size of the corresponding sag stroke of the stirrup electrodes is thereby mainly dependent on the force exerted by the soldering stirrup holder, whereby other factors such as, for example, bent or differently shaped terminal legs of the components or such as a skewed position of the components on the printed circuit board can also have a not inconsiderable influence. These conditions for the sagging or re-sagging of the stirrup electrodes that differ from component to component then necessarily lead to solder points that are differently fashioned, whereby the desired reliability and quality of the soldering is often not achieved.